A light-emitting device comprising two electrodes at least one of which is transparent and a light-emitting layer comprising a luminescent material disposed between the electrodes is called "EL device". The field of use of EL devices is expanding and, currently, they are used in displays of computers and word processors, in annoucement or information boards, in airfield displays and in backlights for automobile gauge panels and the like, among others.
In particular, powder EL elements, which are essential to EL devices and comprise a luminescent material distributed within an organic binder to serve as a dispersion medium and as a dielectric, have advantages in that they are inexpensive and lightweight and that they have a high degree of freedom in shape and form. Future expansion of the field of their application is therefore anticipated.
In powder EL elements, such luminescent materials as zinc sulfide, selenium sulfite, zinc sulfide-selenium sulfide (mixed crystal), calcium sulfide and strontium sulfide are used in the form of luminescent particles, with one or more activators, such as copper, silver, aluminum, manganese, rare earth elements and halogens, added to such base materials as required.
Used as the organic binder are macromolecules having a high dielectric constant, such as the ones listed below:
Japanese Laid-open Patent Application Kokai No. 62482/1980: Cyanoethylcellulose; PA0 Japanese Laid-open Patent Application Kokai No. 111987/1980: Polyimide resins (cyanoethylated polyvinyl alcohol and cyanoethylated saccharose being also mentioned referring to the prior art); PA0 Japanese Laid-open Patent Application Kokai No. 63795/1981: Fluororesins to be used in the molten state, for example polyvinylidene fluoride resins such as polyvinylidene fluoride, trifluoroethylene-vinylidene fluoride copolymer and tetrafluoroethylene-vinylidene fluoride copolymer (mention being made, referring to the prior art, also of epoxy resins, cyanoethyl-cellulose and fluororesins); PA0 Japanese Laid-open Patent Application Kokai No. 189496/1982: Cyanoethylcellulose (mention being made of epoxy resins as the prior art); PA0 Japanese Laid-open Patent Application Kokai No. 151799/1984; Cyanoethylated polysaccharide-glycerol condensates, cyanoethylated polysaccharides and the like, more specifically cyanoethylated glycerol-pullulan condensate, cyanoethylated glycerol-hydroxyethylcellulose condensate, cyanoethylated glycerol-soluble starch condensate, cyanoethylated glycerol-sucrose condensate, cyanoethylated glycerol-polyvinyl alcohol condensate, cyanoethylated sucrose, etc.; fluororesins, such as vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene terpolymer, polytetrafluoroethylene, polychlorotrifluoroethylene, etc.; PA0 Japanese Laid-open Patent Application Kokai No. 81798/1985: Combination of ethylcellulose and terpineol; PA0 Japanese Laid-open Patent Application Kokai No. 211898/1987: Acrylic paints; PA0 Japanese Laid-open Patent Application Kokai No. 234895/1987: Acrylic paints.
Among those mentioned above, epoxy resins and cyanoethylation products have been put to practical use.
However, while future expansion of the fields of application is expected, currently available EL elements are unsatisfactory in brightness or light emission intensity.
Brightness is a function of the voltage and the dielectric constant of the binder (dispersion medium). When the voltage is increased, the brightness also increases. From a practical viewpoint, the voltage to be applied should be not higher than 200 to 300 V. For attaining a satisfactory brightness under such conditions, it is necessary that the binder should have a dielectric constant of not less than 7, desirably not less than 10. Generally, however, most substances having a high dielectric constant are hygroscopic.
Since operating life is greatly influenced by moisture content, it is necessary to minimize the moisture content of the binder and at the same time minimize permeability moisture measures taken to decrease moisture content cause a decrease in dielectric constant.
Powder EL elements, in which particulate zinc sulfide is used as the main component of the luminescent material and an epoxy resin as the organic binder, allow good light transmission and show good electric properties but cannot be said to have sufficient moisture resistance. In particular, luminescent particles consisting mainly of zinc sulfide are very sensitive to moisture and can afford provide only a short operating life. To cope with this problem, measures are known which involve sealing with a plastic film for moisture resistance or removing the moisture adsorbed on the raw material luminescent particles as far as possible, but they are not fully satisfactory.
Organic binders made of a cyanothylated material are poor in heat resistance, and cannot be treated at a temperature necessary for thorough drying. As a result, considerable moisture is retained only a short operating life can be attained, as with epoxy resins.
Accordingly, it is an object of the invention to provide a high-brightness, long-life powder EL element by providing an improved organic binder.